Spiny
waterflea
(Bythotrephes longimanus)
Charles R. O’Neill, Jr.
Invasive Species
Specialist,
New York
Sea Grant
This predatory cladoceran
zooplanktor, commonly known as the spiny waterflea [formerly identified as
Bythotrephes cederstroemi], is a crustacean (a relative of crayfish and
shrimp. A native of the Ponto-Caspian region of Eastern Europe and Western Asia,
Bythotrephes was first found in North America in 1984 in
Lake Huron. By 1987, it had spread into all of the Great Lakes (it
has also been found in inland lakes in
Michigan and southern
Ontario).
It is believed to be an international shipping
ballast water introduction. Its rapid spread throughout the lakes most likely is
the result of currents, inter- and intra-lake ballast transfers and recreational
boating on the lakes. The spiny waterflea grows to about 1 cm long, with a long,
barbed tail spine, which comprises over half the length of the body. This tail
spine frequently snags on lines and downrigger cables of recreational anglers,
in some cases, so many of the crustaceans are attached to the lines that anglers
cannot reel in the lines.
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Bythotrephes
is planktivorous, consuming up to
20 prey zooplanktors per day. One major target species of Bythotrephes is
Daphnia (another small water flea). Research has shown that a dramatic
decrease in Daphnia abundance coincided with the introduction of
Bythotrephes in
Lake Michigan. Density of a native predatory zooplanktor,
Leptodora, also dropped off coincident with the appearance of
Bythotrephes, possibly because Bythotrephes was outcompeting it for
Daphnia. It has been theorized that declines in the abundance of
Daphnia and other Bythotrephes prey may alter the food web in the
Great Lakes, reducing the number of young plankton-eating fish which
survive their first year. Researchers have observed that salmon, walleye and
yellow perch consume Bythotrephes. It is not known, however, how
nutritional this water flea is for fish, given the amount of its mass made up by
exoskeleton and the long tail spine. The ultimate impact of Bythotrephes
on
Great Lakes ecosystems is still unknown. If the water flea is found
to be a preferred (and nutritious) food source for perch and other fishes, its
impact on fish populations may be beneficial. If predation by Bythotrephes
results in reduced populations of preferred prey, such as Daphnia,
the water flea may result in negative consequences to native
Great Lakes fish populations.